To promote the launch of Mass Effect 3, in which gamers must (again) save Earth from alien attackers, Electronic Arts hired ad agency DraftFCB to make a live-action trailer for the launch.

The ad shows San Francisco, Vatican City and parts of China being destroyed by spaceships as humans panic helplessly in the streets. It's intended to make the game feel more like a movie.

But while it's easy to capture computer-generated scenes from a video game and edit them into a 30-second ad, it's harder to make that look like an epic piece of cinematic storytelling that might appeal to a wider audience than sofa-bound young men.

So the agency went to lengths to make the live-action ad as real as possible, and to limit the amount of game-generated animation. Here's how they did it.